Flat light source

ABSTRACT

Two sheets of material at least one of which is transparent, the sheets being overlaid one upon the other and sealed together to form a gas-tight enclosure. One or more gas discharges can be initiated within the enclosure between a pair of electrodes along a predetermined path, the gas discharge path or paths comprising a plurality of adjacent elongate portions extending between and generally parallel to the sheets. The electrodes may be exposed within the enclosure and connected to a DC or AC electrical power source. Alternatively however the electrodes may be electrically insulated from the enclosure but connected to a high frequency AC electrical power source. The enclosure may be filled with low pressure gas in the manner of conventional discharge tubes.

The present invention relates to a flat light source.

Flat light sources are known for illuminating for example displaypanels, the light sources comprising a series of discrete dischargetubes arranged adjacent to one another and energised by an externalcircuitry incorporating ballast or resistive loads. Light sources ofthis type are inevitably bulky and require a space behind the displayscreen which is a of considerable depth. In addition, conventionaldischarge tubes require a high over-voltage to initiate discharge andthe time taken to fully initiate a discharge (the strike time) isconsiderable. The strike time can be reduced by increasing theover-voltage or using internal or external conductive strips extendingalong the length of each discharge tube. The known light sources arehowever difficult to operate over a wide temperature range and if thelight output of the light source is to be controllable complex and bulkyexternal dimming circuitry is required. The known light sources are notgenerally two terminal devices.

Because of the abovementioned disadvantges it has proved difficult toprovide a flat light source of acceptable size for use in for exampleback-lighting liquid crystal displays.

A discharge lamp having a series of parallel elongate dischargecompartments is described in Japanese Patent No. JP-A-57-180067. Thisprovides an output equivalent to a series of separate discharge lamps ina compact structure but does not overcome the problems of slow dischargeinitiation and high minimum overvoltage to initiate a discharge.

Japanese Patent No. JP-A-59-127357 describes a discharge lamp in which asingle discharge compartment is defined in a rectangular enclosure bydividing the enclosure into a series of elongate compartments which areinterconnected end-to-end. A mechanically strong and compact structureresults, but again the problems of high discharge initiation voltagesand slow discharge initiation are not overcome.

It is an object of the present invention to provide a flat light sourcewhich obviates or mitigates the abovementioned problems.

According to the present invention there is provided a flat light sourcecomprising two sheets of material at least one of which is transparent,the sheets being overlaid one upon the other and sealed together to forma gas tight enclosure which is filled with low pressure gas and withinwhich at least one gas discharge can be initiated between a pair ofprimary electrodes along a predetermined elongate path, the gasdischarge path or paths occupying a plurality of adjacent elongatevolumes within the enclosure, characterised in that further electrodesare spaced apart along the length of the or each discharge path, eachfurther electrode being connected to a respective capacitive, resistiveor inductive impedance.

Preferably the primary electrodes are exposed within the enclosure andconnected to a DC or AC electrical power source. Alternatively howeverthe primary electrodes may be electrically insulated from the enclosurebut connected to a high frequency AC electrical power source. Anadditional resistive, capacitive or inductive load may be placed inseries with one or both of the primary electrodes in each pair.

The enclosure may be divided into plurality of parallel channels byspacers positioned between the sheets. Each channel may be associatedwith a respective pair of primary electrodes, or alternatively thechannels may be interconnected end to end to define a single passagewaybetween a single pair of primary electrodes.

The impedances reduce the magnitude of the applied voltage required toinitiate a discharge and can be arranged so as to avoid any need forvariable loads to assist discharge initiation or dimming of the lightoutput. The impedances also have the important practical functions ofstabilising the gas discharges and allowing spacers to be used which donot touch the top and bottom sheets at all points.

Preferably each spacer in section tapers towards the said at least onesheet of material. Alternatively, each spacer is transparent and insection tapers away from the said at least one sheet of material. As afurther alternative, each spacer in section may be curved adjacent eachof the sheets, the surface of each spacer being coated with a reflectivematerial adjacent the sheet remote from the said at least one sheet ofmaterial. A gap may be defined between each spacer and the said at leastone sheet of material. Supports may be interposed between each spacerand the said at least one sheet of material to maintain the said gap.The supports may comprise projections printed on the said at least onesheet of material, of fibres extending across the spacers for example. Adiffusing panel may be supported adjacent the said at least one sheet ofmaterial.

In one arrangement, each further electrode is connected to therespective impedance by a conductor supported on an internal surface ofthe enclosure beneath an insulating layer. Alternatively, each impedanceis supported on an internal surface of the enclosure beneath aninsulating layer, each further electrode being in contact with therespective impedance. Preferably each further electrode is exposed tothe gas in the enclosure through a respective aperture in the saidinsulating layer. An even number of said further electrodes may bespaced apart along the length of the or each said elongate path, eachfurther electrode being connected by the respective impedance to one orother of the primary electrodes, and each further electrode of eachadjacent pair of further electrodes being connected by their respectiveimpedances to different ones of the two primary electrodes. Eachelectrode and impedance may be connected in a circuit which is supporteddirectly on the enclosure formed by the sealed together sheets ofmaterial to define a two-terminal device.

In order to improve the luminous efficiency of the light source, one orall of the surfaces exposed to the gas discharge may be coated withphosphor or fabricated from a phosphor containing material. Surfacesother than that of the transparent sheet may be coated with a reflectivematerial.

Embodiments of the present invention will now be described, by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1 is a plan view of a first primary electrode and enclosurearrangement, the further electrodes not being shown in thisillustration, which is intended to show the basic enclosure and primaryelectrode layout only;

FIG. 2 is a sectional view taken on line 2--2 of FIG. 1;

FIG. 3 illustrates features of a second arrangement incorporating analternative primary electrode structure to that of the arrangement ofFIGS. 1 and 2;

FIG. 4 is a plan view of a third arrangement, the further electrodesagain not being shown;

FIG. 5 is a sectional view on the line 5--5 of FIG. 4;

FIGS. 6 and 7 illustrate alternative spacer structures for use forexample in the arrangement of FIG. 4;

FIG. 8 illustrates a diffuser for use for example in embodiments of thethe invention;

FIG. 9 illustrates the provision of a resistor in series with a primaryelectrode;

FIGS. 10 and 11 show in detail structures incorporating furtherelectrodes with the enclosure in accordance with the present invention;

FIG. 12 illustrates a structure in which an electrode is defined on aside surface of an enclosure defined by an embodiment of the presentinvention;

FIGS. 13 and 14 are circuit diagrams illustrating the connection of anembodiment of the invention to AC and DC power sources respectively; and

FIGS. 15 and 16 illustrate alternative embodiments of the inventionconnected to AC and DC power sources respectively.

Refferring to FIGS. 1 and 2, the illustrated structure comprises a lowersheet of material 1 and an upper transparent sheet of material 2. Thesheets 1 and 2 are spaced apart in parallel and sealed around theirperpheries by a sealing material 3 so as to define a gas-tightenclosure, the enclosure being divided into four parallel channels 4 byspacers 5.

Primary electrodes 6 are exposed within the channels 4 so that anelectrical discharge may be initiated between the pair of electrodespositioned at opposite ends of the respective channel 4.

The channels are filled with a low pressure gas which may be of the typeused in for example conventional discharge tubes and the surfaces of thechannels may be coated with phosphor so as to increase the luminousefficiency of the device. The surfaces of the channels 4 other than theunderside surface of the upper sheet 2 may be coated with a reflectivematerial so as to improve the projection of light through thetransparent upper sheet 2.

When a discharge is initiated in each of the channels 4 simultaneously alarge area flat light source is defined. Such a light source may be usedfor example to back-light a liquid crystal display. The number andlength of channels may be selected as necessary to provide the desiredarea and outline shape for that area.

In the embodiment of FIGS. 1 and 2 the electrodes 6 extend intoelectrical contact with the gas within the channels 4. The electrodes 6may however be electrically insulated from the channels 4 as illustratedin FIG. 3 providing a high frequency signal is applied between theterminals connected to electrodes 6.

Referring now to FIGS. 4 and 5, in the illustrated arrangement a lowersheet 7 is seperated from an upper sheet 8 by series of spacers 9, theupper and lower sheets being sealed together around their peripheries bya seal 10. A single pair of primary electrodes 11 is provided, oneelectrode being positioned at each end of a circuitous single dischargepath defined by the nine channels 12 which are connected end to end byvirtue of the spacers 9, one end of each spacer stopping short of theadjacent portion of the seal 10.

As in the arrangement of FIGS. 1 and 2 the channels 12 are filled with alow pressure gas. When a discharge is initiated between the twoelectrodes 11 light is emitted from each of the channels 12 and thus alarge area flat light source is defined.

FIG. 5 illustrates the connection of the electrode 11 to an externalterminal 13. The electrode 11 is deposited on the upper surface of thelower sheet 7 and connected to the terminal 13 which is also depositedon the lower sheet 7. An insulating layer 14 is then laid over thejunction between the electrode 11 and the terminal 13 and the seal 10 isthen positioned so that the seal 10 and insulating layer 14 preventdirect electrical contact between the terminal 13 and the gas within thechannel 12.

In the arrangement of FIGS. 4 and 5 the spacers 9 may be simple stripsof rectangular cross section as shown in FIG. 6. With such anarrangement however the top edges of the spacers 9 are visible when thelight source is viewed through the upper sheet 8. To reduce the visibleeffects of the positioning of the spacers 9 various techniques may beused. For example the spacers 9 may be made of a transparent materialsuch as glass and may be made as narrow as possible adjacent the uppersheet 8. For example spacers having a triangular cross section ratherthan the rectangular cross section as shown in FIG. 6 could be used. Asan alternative when the spacers 9 are fabricated from a transparentmaterial the spacers 9 may be made relatively wide close to the uppersheet 8 so that light transmitted into the spacer 9 is direct towardsthe upper sheet 8. In such an arrangement the spacer 9 could have atriangular cross section with one face of the triangle flat against theunderside of the upper sheet 8. Other techniques for adjusting thevisual impact of the spacers 9 may be used, for example by surfacetreatment, adjustment to the detailed geometrical shape, construction ofthe spacers from semi-transparent or diffusing material etc. A furthermethod is to curve the surfaces of spacers 9, adjacent to the upper andlower blocks, and to coat the surface adjacent to the lower block with areflective material.

FIG. 7 shows an alternative spacer structure which substantiallyeliminates the visual impact of the spacers when the light source isviewed through the upper transparent sheet 8. In the arrangement of FIG.7 each spacer comprises a portion 15 of triangular cross section whichspaced from the upper sheet 8. Thus the spacers are not in contact withthe upper sheet 8. Small "pips" 16 of a dielectric material are screenprinted on the underside of the upper sheet 8 and contact the upperedges of the spacer portions 15. The functional effect of the pips 16may be achieved in other ways, for example by using discrete thinsupport elements such as glass fibers extending across the top of thespacer portions 15.

As a further alternative, the pips 16 may be simply omitted providingthe upper sheet 8 is sufficiently rigid to prevent it being suckeddownwards in an irregular manner by the differential pressure across itsthickness.

FIG. 8 illustrates a further structure which produces a more uniformlight source. In the arrangement of FIG. 8 the upper sheet 8 of thelight source (the lower sheet 7 not being shown) supports a diffusingpanel 17 on edge spacers 18. This produces a very uniform light sourcebut at the expense of increasing the overall thickness of the device.Alternatively the diffuser may be in contact with the upper block orincorporated as part of the upper block.

It will be noted from the above that it is not necessary for the spacers9 to be sealed at their upper and lower edges to the upper and lowersheets 7 and 8. Depending on th length, width and overall dimensions ofthe light source the barriers represented by the spacers betweenadjacent discharge channels may vary considerably. It is simplynecessary to provide sufficiently extensive barriers to ensure that adischarge in one channel does not prevent the initiation of a dischargein an adjacent channel.

In the arrangement of FIGS. 4 and 5 the electrodes 11 are directlyconnected to the terminals 13. Accordingly if any ballast resistors arerequired these must be provided external to the enclosure defined by thelight source. As an alternative however and as illustrated in FIG. 9 aresistor 19 may be deposited on the lower sheet 7 so as to be connectedin series between the electrode 11 and the terminal 13. The resistor 19is electrically insulated from the discharge channel 12 by the layer 14of insulating material. It will of course be appreciated that theresistor 19 could be placed beneath the seal 10 or alternatively on thesurface of the lower sheet 7 external to the seal 10.

The voltage required to initiate a discharge in one of the channels 12is reduced and the overall stability of the gas discharge or dischargesis improved in accordance with the present invention by arranging aseries of further electrodes along the length of the channels. Such anarrangement is illustrated in FIG. 10 in which the top surface of thelower sheet 7 is shown as supporting between the electrodes 11 a seriesof secondary electrodes 20 each individually connected by a series ofelectrical conductors 21 and 22 to an external resistive, inductive orcapacitive ballast (not shown). It will be appreciated that theelectrodes 20 and 11 are exposed to the gas discharge through respectiveholes in the adjacent dielectric. The electrodes are not located in openchannels in the dielectric. The electrical conductors 21 and 22 areelectrically insulated from the gas in the channel with which they areassociated by a layer of insulating material 23. As described in furtherdetail below, the electrical conductors 21 and 22 linked electricallytogether in two groups via respective series ballast impedances andconductors external to the discharge enclosure. It is possible to placea plurality of electrodes 20 across the width of each channel so thateach group of electrodes extending across the width of the channel canbe considered as constituting a single further electrode in the seriesof such electrodes spaced apart between the primary electrodes 11.Detailed circuits for use with a structure such as that illustrated inFIG. 10 are described below.

Referring now to FIG. 11, this illustrates a structure which enables animpedance in the form of a resistive element 24 to be placed on thelower sheet 7 in series with each electrode 20 and each electricalconductor 21 or 22. The electrical conductors 21 or 22 connect all theresistive elements 24 in rows extending perpendicular to the channels.The resistors 24 are electrically insulated from discharges by the layerof insulating material 23. The electrical supply to the device will beappreciated from the detailed circuits desrcibed below.

The formation of the electrodes. resistors, conductors and insulationlayers on the lower sheet 7 may be achieved in any convenient manner.For example components may be deposited by printing, vapour depositionor any other conventional techniques including physical insertion ofdiscrete components where appropriate.

Preferably the lower sheet is coated with phosphor. It may be necessaryto clean the electrode surfaces after the phosphor coating is applied.The electrodes may be cleaned more easily by increasing the thickness ofthe electrodes to make them project above the level of the insulatingmaterial covering the conductors. Alternatively the cleaning process maybe eliminated by placing the electrodes beneath the spacer elements,such that the electrodes remain in electrical contact with the gas, withthe spacers having a maximum width in the vicinity of the electrodes.The phosphor coating may then be sprayed in such a fashion that theelectrodes are shielded from any phosphor coating.

In the embodiments described above all the various components have beensupported on the lower sheet 7. This means that the area used by thesecomponents is relatively large when the device is viewed through theupper transparent sheet 8. This area may be reduced however by usingsurfaces perpendicular to the two sheets to support the variouselectrodes and other electrical components. Such an arrangement is shownin FIG. 12. Electrodes 25 are for example silk screed printed on theupper sheet 8 and lower sheet 7 and on to the surface 26 perpendicularthereto. An internal conductive connection 27 is made between theelectrode 25 to the lower sheet and the perpendicular surface 26. Theelectrodes on the upper and lower sheets and resistors 28 are connectedtogether externally by linking conductors 29. A single load resistorcould be used common to both the upper and lower block electrodes.

FIGS. 13 and 14 illustrate the circuit of an embodiment constructed tocomprise the components described with reference to FIGS. 9 and 10.Ballast impedance resistors 30 are provided as described with referenceto but not illustrated in FIG. 10. The perimeter of the gas filledenclosure is indicated by dotted line 31 and the position of spacersdividing the enclosure into three independent discharge channels areindicated by dotted lines 32. FIG. 13 illustrates the circuit for ACenergisation and FIG. 14 the circuit for DC energisation.

FIGS. 15 and 16 illustrate the circuits of embodiment similar to thoseillustrated in FIGS. 13 and 14 but incorporating the resistors 24 asillustrated in FIG. 11. Current limiting resistors 33 are also provided.The perimeter of the enclosure is indicated by dotted lines 34 andspacers seperating the enclosure into three independent dischargechannels are indicated by dotted lines 35. The current limitingresistors 33 could, of course, be incorporated within the perimeter ofthe enclosure as could the external conductors shown in FIGS. 15 and 16thereby producing a two terminal device.

I claim:
 1. A flat light source comprising two sheets of material atleast one of which is transparent, the sheets being overlaid one uponthe other and sealed together to form a gas tight enclosure which isfilled with low pressure gas and within which at least one gas dischargecan be initiated between a pair of primary electrodes along apredetermined elongate path, the gas discharge path or paths occupying aplurality of adjacent elongate volumes within the enclosure,characterised in that further electrodes are spaced apart along thelength of the or each discharge path, each further electrode beingconnected to a respective capacitive, resistive or inductive impedance.2. A flat light source according to claim 1, wherein the primaryelectrodes are exposed within the enclosure and arranged for connectionto a DC or AC electrical power source.
 3. A flat light source accordingto claim 1, wherein the primary electrodes are electrically insulatedfrom the enclosure and arranged for connection to a high frequency ACelectrical power source.
 4. A flat light source according to any claim 1wherein the enclosure is divided into a plurality of parallel channelsby spacers positioned between the sheets, each channel defining a saidelongate volume.
 5. A flat light source according to claim 4, whereineach channel is associated with a respective pair of primary electrodes.6. A flat light source according to claim 4, wherein a plurality of thechannels are interconnected end to end to define a single passagewaybetween a single pair of primary electrodes.
 7. A flat light sourceaccording to claim 4, 5 or 6, wherein each spacer in section taperstowards the said at least one sheet of material.
 8. A flat light sourceaccording to claim 4, 5 or 6, wherein each spacer is transparent and insection tapers away from the said at least one sheet of material.
 9. Aflat light source according to claim 4, 5 or 6, wherein each spacer insection is curved adjacent each of the sheets, the surface of eachspacer being coated with a reflective material adjacent the sheet remotefrom the said at least one sheet of material.
 10. A flat light sourceaccording to claims 4, 5, or 6, wherein a gap is defined between eachspacer and the said at least one sheet of material.
 11. A flat lightsource according to claim 10, comprising support means interposedbetween each spacer and the said at least one sheet of material tomaintain the said gap.
 12. A flat light source according to claim 11,wherein the support means comprise projections printed on the said atleast one sheet of material.
 13. A flat light source according to claim11, wherein the support means comprise fibres extending across thespacers.
 14. A flat light source according to claim 1, comprising adiffusing panel supported adjacent the said at least one sheet ofmaterial.
 15. A flat light source according to claim 1, wherein eachfurther electrode is connected to the respective impedance by aconductor supported on an internal surface of the enclosure beneath aninsulating layer.
 16. A flat light source according to claim 1, whereineach said impedance is supported on an internal surface of the enclosurebeneath an insulating layer, each further electrode being in contactwith the respective impedance.
 17. A flat light source according toclaim 15 or 16, wherein each further electrode is exposed to gas in theenclosure through a respective aperture in the said insulating layer.18. A flat light source according to claim 1, comprising a plurality ofsaid further electrodes spaced apart along the length of the or eachsaid elongate path, each further electrode being connected by itsrespective impedance to one or other of the primary electrodes, and eachfurther electrode of each adjacent pair of further electrodes beingconnected by the respective impedances to different ones of the twoprimary electrodes.
 19. A flat light source according to claim 1,wherein an additional resistive, capacitive or inductive load isconnected in series with one or both of the or each pair of primaryelectrodes.
 20. A flat lilght source according to claim 19, wherein eachelectrode and impedance is connected in a circuit which is supporteddirectly on the enclosure formed by the sealed together sheets ofmaterial to define a two-terminal device.
 21. A flat light sourceaccording to claim 1, wherein at least one of the surfaces exposed tothe or each gas discharge is coated with phosphor or fabricated from aphosphor containing material.
 22. A flat light source according to claim1, wherein surfaces exposed to the low pressure gas other than that ofthe transparent sheet are coated with a reflective material.